The present invention relates to a tensile impact apparatus that is designed to give dynamic stress-strain curves of uniaxial strip specimens and force-extension curves for notched specimens and further provides for visual observation and study of fracture characteristics for a rubber specimen undergoing tensile impact loading.
Various types of hi-speed tensile impact test equipment are currently available, including open-and closed-loop hydraulic systems; pneumatic systems; pneumatically actuated, hydraulically damped systems; resonant beam systems; and pendulum impact systems. Other types of impact test equipment, such as those employing fly wheels, smokeless powder-driven actuators, and dual-opposing actuators are available for special test applications; however, these types of test equipment are typically custom made for such applications, and the demand for such equipment is therefore quite limited. Furthermore, they only provide tensile strength and fracture strain and do not give dynamic stress-strain curves at very high rates. While present-day apparatus, such as the tensile Split Hopkinson Pressure Bar and expansion ring tests, can provide dynamic stress-strain curves at very high rates, they are not designed to monitor how the specimen fractures. In distinction, the tensile impact apparatus described herein gives both dynamic stiffness and strength characteristics of rubber and is capable of monitoring specimen fracture.
Until recently, there has been very little need for understanding the tensile impact response of an elastomer (rubber-like) material at large strains and high strain rates. Typical elastomeric structures, such as vibration isolators, shock pads and base isolation bearings, carry loads in compression and/or shear and operate in regimes where the engineering strains are below 100%. However, as uses for polymeric materials become more widespread and diverse, the ability to characterize tensile behavior at large strains and high strain rates will be very useful. For example, the Air Force Research Laboratory (AFRL) at Tyndall Air Force Base (AFB) discovered that polyurethane elastomer coatings on concrete blocks, reinforced concrete, and trailer walls could offer significant protection for occupants when the walls were subjected to air blast or explosive loading. T. R. Anderl, “Space-age coating protecting against terrorism,” Air Force Research Laboratory Materials and Manufacturing Directorate, 10 Feb. 2003. The hyper-viscoelastic behavior of the polyurethane rubber allowed the coated wall to flex and absorb blast energy. The elastomer itself also provided a nesting zone for blast fragments and prevented harmful projectiles from entering buildings. In order for the Air Force to understand the blast protection effectiveness of elastomer coatings, it is necessary or at least helpful to characterize tensile behavior of elastomers at large strains and high strain rates (10-103 s−1).
As another example, rubber may break under very high strain rates in tire applications. For example, a faulty tire on a car traveling at 55 mph (89 km/h) may suddenly break at a shear strain rate of 404 s−1 when it hits a bump in the road. Unfortunately, currently accepted test methods for determining the mechanical properties of rubber in the tire industry cannot provide adequate data to predict this type of failure. Creep and relaxation tests determine rubber properties under quasi-static loading, while vibration and rebound tests determine loss and storage modulus for vibratory loads. The vibration and rebound tests may be capable of reaching high rates or frequencies but are limited by low strains (well below the tensile fracture strain of rubber).
Dynamic material properties for elastomers are often specified for structural response in the frequency rather than the time domain. Vibration experiments are used to find complex modulus in the frequency domain and this data is often used to design shock absorbers and base isolation bearings. J. M. Kelly, “Earthquake Resistant Design with Rubber,” Springer-Verlag, London, 1997. However, the concept of a complex modulus is based on linear viscoelastic material behavior, i.e., material for which stress is directly proportional to strain and strain rate. Frequency domain material properties are therefore limited to applications where strains are small and stress is approximately linear with strain and the strain rate. Frequency domain material properties become irrelevant if the material exhibits nonlinear elastic behavior or is subjected to large strains. Under tension, elastomers are not only nonlinear elastic but also hyperelastic, i.e., they can stretch 300-500% before breaking. Clearly, new dynamic material properties are needed to characterize elastomeric structures undergoing high strain rates and nonlinear, hyperelastic behavior.
Thus, there exist a need to develop a tensile impact apparatus that is designed to give dynamic stress-strain curves of a rubber specimen undergoing tensile impact loading. Particularly, there is a need for an apparatus that is capable of achieving strains sufficient to fracture virtually any elastomer sample. The data provided by the tensile impact apparatus would enable one to predict tensile fracture of rubber components under shock or impact loads.